Month: September 2016

Summer is ending in the northern hemisphere. That’s good news for sky watchers because autumn is “aurora season.” Autumn is special in part because lengthening nights and crisp pleasant evenings tempt stargazers outside; they see things they ordinarily wouldn’t. But there’s more to it than that: autumn really does produce a surplus of geomagnetic storms–almost twice the annual average.

In fact, both spring and autumn are good aurora seasons. Winter and summer are poor. This is a puzzle for researchers because auroras are triggered by solar activity. The Sun doesn’t know what season it is on Earth–so how could one season yield more auroras than another?

To understand the answer, we must first understand what causes auroras themselves.

Auroras appear during geomagnetic storms–that is, when Earth’s magnetic field is vibrating in response to a solar wind gust. Such gusts pose no danger to people on the ground because our magnetic field forms a bubble around Earth called the magnetosphere, which protects us. The magnetosphere is filled with electrons and protons. “When a solar wind gust hits the magnetosphere, the impact knocks loose some of those trapped particles,” explains space physicist Tony Lui of Johns Hopkins University. “They rain down on Earth’s atmosphere and cause the air to glow where they hit–like the picture tube of a color TV.”

Below: Still frames from a digital movie show how solar wind gusts rattle Earth’s magnetosphere and trigger auroras. Click to view the 750 kb Quicktime animation created by Digital Radiance, Inc.

Some solar wind gusts (“coronal mass ejections”) are caused by explosions near sunspots, others are caused by holes in the Sun’s atmosphere (“coronal holes”) that spew solar wind streams into interplanetary space. These gusts sweep past Earth year-round, which returns us to the original question: why do auroras appear more often during spring and autumn?

The answer probably involves the Sun’s magnetic field near Earth. The Sun is a huge magnet, and all the planets in the solar system orbit within the Sun’s cavernous magnetosphere. Earth’s magnetosphere, which spans about 50,000 km from side to side, is tiny compared to the Sun’s.

The outer boundary of Earth’s magnetosphere is called the magnetopause–that’s where Earth’s magnetic field bumps into the Sun’s and fends off the solar wind. Earth’s magnetic field points north at the magnetopause. If the Sun’s magnetic field tilts south near the magnetopause, it can partially cancel Earth’s magnetic field at the point of contact.

“At such times the two fields (Earth’s and the Sun’s) link up,” says Christopher Russell, a Professor of Geophysics and Space Physics at UCLA. “You can then follow a magnetic field line from Earth directly into the solar wind.” Researchers call the north-south component of the Sun’s nearby magnetic field “Bz” (pronounced “Bee-sub-Zee”). Negative (south-pointing) Bz‘s open a door through which energy from the solar wind can reach Earth’s inner magnetosphere. Positive (north-pointing) Bz‘s close the door.

In the early 1970’s Russell and colleague R. L. McPherron recognized a connection between Bz and Earth’s changing seasons. “It’s a matter of geometry,” explains Russell. Bz is the component of the Sun’s magnetic field near Earth which is parallel to Earth’s magnetic axis. As viewed from the Sun, Earth’s tilted axis seem to wobble slowly back and forth with a one-year period. The wobbling motion is what makes Bz wax and wane in synch with the seasons.

In fact, Bz is always fluttering back and forth between north and south as tangled knots of solar magnetic field drift by Earth. What Russell and McPherron realized is that the average size of the flutter is greatest in spring and fall. When Bz turns south during one of those two seasons, it really turns south and “opens the door wide” for the solar wind.

Mystery solved? Not yet. In a Geophysical Research Letter (28, 2353-2356, June15, 2001), Lyatsky et al argued that Bz and other known effects account for less than one-third of the seasonal ups-and-downs of geomagnetic storms. “This is an area of active research,” remarks Lui. “We still don’t have all the answers because it’s a complicated problem.”

Aug. 30, 2016: Researchers have long known that solar activity and cosmic rays have a yin-yang relationship. As solar activity declines, cosmic rays intensify. Lately, solar activity has been very low indeed. Are cosmic rays responding? The answer is “yes.” Spaceweather.com and the students of Earth to Sky Calculus have been using helium balloons to monitor cosmic rays in the stratosphere over California. Their latest data show an increase of almost 13% since 2015.

Cosmic rays, which are accelerated toward Earth by distant supernova explosions and other violent events, are an important form of space weather. They can seed clouds, trigger lightning, and penetrate commercial airplanes. Furthermore, there are studies ( #1, #2, #3, #4) linking cosmic rays with cardiac arrhythmias and sudden cardiac death in the general population.

Why are cosmic rays intensifying? The main reason is the sun. Solar storm clouds such as coronal mass ejections (CMEs) sweep aside cosmic rays when they pass by Earth. During Solar Maximum, CMEs are abundant and cosmic rays are held at bay. Now, however, the solar cycle is swinging toward Solar Minimum, allowing cosmic rays to return. Another reason could be the weakening of Earth’s magnetic field, which helps protect us from deep-space radiation.

The data points in the graph above correspond to the peak of the Reneger-Pfotzer maximum, which lies about 67,000 feet above central California. When cosmic rays crash into Earth’s atmosphere, they produce a spray of secondary particles that is most intense at the entrance to the stratosphere. Physicists Eric Reneger and Georg Pfotzer discovered this maximum using balloons in the 1930s and it is what we are measuring today.